/*
 * Copyright (C) 2015 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.hash;

import com.google.caliper.BeforeExperiment;
import com.google.caliper.Benchmark;
import com.google.caliper.Param;

import java.security.MessageDigest;
import java.util.Arrays;
import java.util.Random;

/**
 * Benchmarks for comparing the various {@link HashCode#equals} methods.
 *
 * <p>Parameters for the benchmark are:
 *
 * <ul>
 *   <li>size: the length of the byte array to hash
 *   <li>whereToDiffer: where in the array the bytes should differ
 *   <li>equalsImpl: which implementation of array equality to use
 * </ul>
 *
 * <p><b>Important note:</b> the primary goal of this benchmark is to ensure that varying {@code
 * whereToDiffer} produces no observable change in performance. We want to make sure that the array
 * equals implementation is *not* short-circuiting to prevent timing-based attacks. Being fast is
 * only a secondary goal.
 *
 * @author Kurt Alfred Kluever
 */
public class HashCodeBenchmark
{

    // Use a statically configured random instance for all of the benchmarks
    private static final Random random = new Random(42);

    @Param({"1000", "100000"})
    private int size;

    @Param
    WhereToDiffer whereToDiffer;

    @Param
    EqualsImplementation equalsImpl;

    private enum WhereToDiffer
    {
        ONE_PERCENT_IN,
        LAST_BYTE,
        NOT_AT_ALL;
    }

    private enum EqualsImplementation
    {
        ANDING_BOOLEANS
                {
                    @Override
                    boolean doEquals(byte[] a, byte[] b)
                    {
                        if (a.length != b.length)
                        {
                            return false;
                        }
                        boolean areEqual = true;
                        for (int i = 0; i < a.length; i++)
                        {
                            areEqual &= (a[i] == b[i]);
                        }
                        return areEqual;
                    }
                },
        XORING_TO_BYTE
                {
                    @Override
                    boolean doEquals(byte[] a, byte[] b)
                    {
                        if (a.length != b.length)
                        {
                            return false;
                        }
                        byte result = 0;
                        for (int i = 0; i < a.length; i++)
                        {
                            result = (byte) (result | a[i] ^ b[i]);
                        }
                        return (result == 0);
                    }
                },
        XORING_TO_INT
                {
                    @Override
                    boolean doEquals(byte[] a, byte[] b)
                    {
                        if (a.length != b.length)
                        {
                            return false;
                        }
                        int result = 0;
                        for (int i = 0; i < a.length; i++)
                        {
                            result |= a[i] ^ b[i];
                        }
                        return (result == 0);
                    }
                },
        MESSAGE_DIGEST_IS_EQUAL
                {
                    @Override
                    boolean doEquals(byte[] a, byte[] b)
                    {
                        return MessageDigest.isEqual(a, b);
                    }
                },
        ARRAYS_EQUALS
                {
                    @Override
                    boolean doEquals(byte[] a, byte[] b)
                    {
                        return Arrays.equals(a, b);
                    }
                };

        abstract boolean doEquals(byte[] a, byte[] b);
    }

    private byte[] testBytesA;
    private byte[] testBytesB;

    @BeforeExperiment
    void setUp()
    {
        testBytesA = new byte[size];
        random.nextBytes(testBytesA);
        testBytesB = Arrays.copyOf(testBytesA, size);
        int indexToDifferAt = -1;
        switch (whereToDiffer)
        {
            case ONE_PERCENT_IN:
                indexToDifferAt = (int) (size * 0.01);
                break;
            case LAST_BYTE:
                indexToDifferAt = size - 1;
                break;
            case NOT_AT_ALL:
        }
        if (indexToDifferAt != -1)
        {
            testBytesA[indexToDifferAt] = (byte) (testBytesB[indexToDifferAt] - 1);
        }
    }

    @Benchmark
    boolean hashFunction(int reps)
    {
        boolean result = true;
        for (int i = 0; i < reps; i++)
        {
            result ^= equalsImpl.doEquals(testBytesA, testBytesB);
        }
        return result;
    }
}
